The present invention relates to a method of printing a pattern on a wafer and an apparatus for carrying out such pattern printing method.
A typical semiconductor manufacturing process includes one step, generally known as lithography, in which a pattern is printed on a circuit pattern which has already been formed on a wafer in a previous step in such a manner that these two patterns are precisely aligned with each other. As apparatuses for such pattern printing, reduction projection aligners and X-ray aligners have heretofore been employed (see e.g., U.S. Pat. No. 4,380,395).
FIG. 1 is a schematic sectional view showing an essential part of a conventional pattern printing apparatus using X-rays as an exposure source. In the figure: the reference numeral 6 denotes a stage; 20, a drive unit for driving the stage 6; 24, a pattern detector: 1, a wafer placed on the stage 6; 2, a circuit pattern which has already been formed on the wafer 1 in a previous step; 3, a photoresist layer coated on the surface of the wafer 1; 21a, 21b, target marks provided on the surface of the wafer 1 which is printed with the pattern 2; 22, a photo mask; and 23a, 23b, opening patterns for alignment provided in the photo mask 22. An X-ray source is not shown in FIG. 1.
With such pattern printing apparatus, a pattern on the photo mask 22 is printed on the wafer 1 in the manner described below. The stage 6 is moved by the drive unit 20 while detecting the target marks 21a, 21b and the opening patterns 23a, 23b using the pattern detector 24 so that the target marks 21a and 21b are positioned at the respective centers of the opening patterns 23a and 23b, thereby precisely aligning the pattern 2 and the pattern on the photo mask 22 with each other. Then, the pattern detector 24 is moved out of an exposure range, and exposure is effected by an exposure unit (not shown).
The above-described conventional pattern printing method suffers, however, from the following problem. Since a multiplicity of layers are formed over the target marks 21a and 21b as the semiconductor manufacturing process proceeds, the configurations of the target marks 21a and 21b are deteriorated. In addition, since the target marks 21a and 21b must be observed through the photoresist layer 3 coated thereon, it is impossible to obtain a detection signal with excellent or high S/N ratio. Therefore, the degree of accuracy in alignment is lowered as the semiconductor manufacturing process proceeds. Further, the prior art involves the need to move the pattern detector 24 out of the exposure light path every time pattern printing is carried out.